1. Field of the Invention
This invention relates to polyurethanes containing acryloyl groups which may be converted into crosslinked products by radiation curing and to a process for their production.
2. Description of the Prior Art
Polyurethanes containing acryloyl groups which are obtained as a reaction product of inter alia polyisocyanates and hydroxyalkyl acrylates, as described for example in DE-OS 1,644,798, DE-OS 2,115,373 or DE-OS 2,737,406, are known. They are cured by high-energy radiation, such as UV radiation or electron beams, and are used, for example, for coating wood or for the production of coatings for graphic purposes.
Various polyisocyanates may be used for the production of the urethane acrylates. If the production of non-yellowing paint films is an objective, (cyclo)aliphatic polyisocyanates are used for the production of the urethane acrylates. Although hexamethylene diisocyanate is also mentioned as a suitable starting diisocyanate in DE-OS 2,115,373, it is clear that this is a purely hypothetical disclosure because it has never been possible to use this inexpensive diisocyanate due to the crystallinity of the urethane acrylates formed which is a disadvantage to their use as binders for coating compositions.
One of Applicants' earlier patent applications (German patent application P 40 27 743.7, U.S. Pat. No. 5,128,432) describes ester alcohol-modified urethane acrylates based on hexamethylene diisocyanate which are liquid at room temperature after their production.
An object of the present invention is to provide another method of obtaining aliphatic polyurethanes containing acryloyl groups using hexamethylene diisocyanate (HDI) as the primary starting diisocyanate which are liquid, even at temperatures of 0.degree. C., and crosslink rapidly under UV irradiation.
This object may be achieved in accordance with the process according to the invention for the production of aliphatic polyurethanes containing acryloyl groups which is described in detail hereinafter.